Proterozoic to Phanerozoic Tectonism in Southwestern Montana Basement Ranges Constrained by Low Temperature Thermochronometric Data. Issue 11 (10th November 2021)

• Record Type: Journal Article
• Authors: Kaempfer, Jenna M.; Guenthner, William R.; Pearson, David M.
• Is Part Of: Tectonics. Volume 40:Issue 11(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-11-10
• Subjects: great unconformity -- HeFTy -- Laramide tectonics -- thermochronology -- zircon (U‐Th)/He; Geology, Structural -- Periodicals; 551.8
• DOI: 10.1029/2021TC006744
• Languages: English
• ISSNs: 0278-7407

Abstract

Abstract: Crystalline basement rocks of southwestern Montana have been subjected to multiple tectonothermal events since ∼3.3 Ga: the Paleoproterozoic Big Sky/Great Falls orogeny, Mesoproterozoic extension associated with Belt‐Purcell basin formation, Neoproterozoic extension related to Rodinia rifting, and the late Phanerozoic Sevier‐Laramide orogeny. We investigated the long‐term (>1 Ga), low‐temperature (erosion/burial within 10 km of the surface) thermal histories of these tectonic events with zircon and apatite (U‐Th)/He thermochronology. Data were collected across nine sample localities ( n = 55 zircon and n = 26 apatite aliquots) in the northern and southern Madison ranges, the Blacktail‐Snowcrest arch, and the Tobacco Root uplift. Our zircon (U‐Th)/He data show negative trends between single aliquot date and effective uranium (a radiation damage proxy), which we interpreted with a thermal history model that considers the damage‐He diffusivity relationship in zircon. Our model results for these basement ranges show substantial cooling from temperatures above 400°C to near surface conditions between 800 and 510 Ma. Subsequent Phanerozoic exhumation culminated by ∼75 Ma. Late Phanerozoic cooling is coincident with along‐strike Sevier belt thin‐skinned thrusting in southeastern Idaho, and older than exhumation in basement‐involved uplifts of the Wyoming Laramide province. Our long‐term, low‐temperature thermal record for these southwestern Montana basement ranges shows that: (a) these basement blocks have experienced multiple episodes of upper crustal exhumation and burial since Archean time, possibly influencing Phanerozoic thrust architecture and (b) the late Phanerozoic thick‐skinned thrusting recorded by these rocks is among the earliest thermochronologic records of Laramide basement‐involved shortening and was concomitant with Sevier belt thin‐skinned thrusting. Key Points: Thermal history modeling of zircon (U‐Th)/He data reveals >1 Ga time‐temperature paths of basement rocks in southwestern Montana These models show cooling from temperatures >300°C to near surface conditions between 800 and 510 Ma Results suggest recurrent tectonism from the Paleoproterozoic to late Phanerozoic with implications for Rodinia rifting‐related exhumation